High frequency radio frequency (RF) communications are becoming more and more prevalent in the world today. Products touting wireless RF communication links are becoming increasingly popular among consumers. A multitude of new products including redesigned existing ones are being built with wireless RF links today. Most RF communication circuits employ some form of resonant circuitry in their transceivers. Due to the explosive consumer demand for products sporting wireless communication links there is a need for low cost, high accuracy and high reliability filters that are suitable for mass manufacture using conventional techniques.
RF filters are necessary circuits in transmitters and receivers that operate in a both wireless and non-wireless, i.e., cable, environments. In a transmitter, the amount of suppression that an RF filter must provide is determined by regulatory requirements or by the amount of interference that the transmitter might cause as a result of unwanted spectral components. RF filters are even more essential in receivers of communications systems especially when the communications system is wireless and is likely to suffer from reception of interfering signals in addition to the normal reception noise. In a receiver, the quality of the filtering dramatically effects the reception performance, especially when considering certain types of interference. A particular receiver may deliver a much lower output quality (i.e., higher error rate in a digital receiver, or large signal distortions in an analog receiver) if the frequency response of its filters is compromised.
When defining the filtering requirements in a receiver, the following factors should be considered: (1) the frequency band in which the receiver is to operate, (2) the frequency conversions and IF to be used, (3) the spectrum of the modulated signal to be demodulated by the receiver and (4) the nature of any interfering signals to be encountered and the associated rejection requirements.
The filter preceding the demodulator is normally the narrowest and should allow a minimum amount of additive noise and interference to enter the demodulator. Its bandwidth is normally close to that of the modulated signal for which the receiver was designed. The selectivity of the receiver, i.e., the rejection of adjacent frequencies which may cause jamming or performance degradation, is determined by the steepness of the frequency response curve of this filter.
In a single frequency receiver, such as in the case of pagers, the narrow filter may precede most of the electronic circuitry of the receiver, thus reducing the possibility of intermodulation products being generated within the receiver. In receivers intended for an entire band of frequencies, from which one frequency among many is in use at any one time, the filtering at the input of the receiver usually has a bandwidth at least as wide as that of the entire band used. These filters cannot provide rejection for in band interference and usually do not have significant attenuation even for out of band frequencies which are close to the edges of the band. In such receivers, the IF filtering provides effective rejection of such interference as long as inband intermodulation products are not generated before the signal is filtered by the narrow IF filtering.
The wide filters located at the input to the receiver are intended to provide image rejection and out of band interference rejection, which is effective for signals sufficiently far from the edges of the frequency band. A typical filter, e.g., surface acoustic wave (SAW) filter, for use in the 900 MHz ISM band, for example, costs over $1.50 and has a frequency response with limited out of band attenuation.
Most RF filters and circuits for communication applications make use of one or more inductors in their design. Previously, these were lumped inductors or printed inductors which have been formed on printed circuit boards using a variety of techniques such as stripline, microstrip, slotline, etc. Inductors formed using any of these techniques are typically constructed in the form of a planar spiral with the spiral being circular or square in shape. A disadvantage, however, of forming inductors, such as microstrip inductors, on printed circuit boards is that they are very sensitive to the characteristics of the printed circuit board material. The characteristics of the printed circuit board material directly affect the characteristics and performance of the inductors formed thereon. Parameters of the PCB material such as thickness and dielectric constant affect the characteristics of the inductor. The sensitivities of microstrip inductors to the dielectric constant of the printed circuit board material causes variations in the resultant self resonant frequency. In addition, variations in the thickness of the PCB material causes variations in the value of the inductance which results in frequency response errors of any filter constructed therefrom.
Another disadvantage of constructing printed inductors on printed circuit boards using traditional techniques is that the repeatability of the value of the inductance is very low. As described above, the characteristics of the inductor are very sensitive to the parameters of the printed circuit board material. In addition, most printed circuit inductors constructed utilizing conventional techniques have limited values of the quality factor Q of the inductor. This is due to the nature of the conventional inductor which is constructed having a ground plane. Further, since the ground plane is separated from the printed inductor traces by the printed circuit board material there is typically significant parasitic capacitance between the inductor and the underlying ground plane. In some applications, this parasitic capacitance can be problematic because it causes a reduction in the self resonant frequency of any LC combination formed using the inductor.
An alternative to using printed circuit inductors, such as microstrip and stripline inductors, is to use discrete inductor elements. A disadvantage, however, of using discrete elements is the high cost typically associated with high Q lumped coils.
An inductor constructed on a semiconductor substrate is known in the prior art. U.S. Pat. No. 5,539,241, issued to Abidi et al., discloses an integrated circuit having a monolithic passive component such as an inductor which is constructed so as to be suspended over a pit in the substrate. Suspending the inductor over a pit serves to reduce parasitic capacitance and enhance the performance of the inductor.